Chippers are used for the production of chips e.g. from logs for pulping or for use as fuel. The chipper knives are subject to heavy wear due to the high utilization rate of the chipping plants and to the presence of detrimental impurities in the wood material being chipped. Thus, replacing the chipper knives is an operation that is normally repeated daily or once during each working shift.
In disc chippers employing previously known knife holding systems, the knife is detached and attached manually by loosening or tightening the knife holding screws. These generate a force that holds the knife or knife holding element in place. Due to the large number of knife holding screws (about 60 . . . 120 pcs for each chipper disc), changing the knives is an operation that requires plenty of work and time. Moreover, to loosen and tighten the screws, heavy wrenches are needed. Replacing the knives in a chipper disc normally requires about 1 h of time, which means about 250 h of work in a year. Thus, at the yearly level, the replacement of the chipper knives is the biggest factor reducing the utilization rate of the chipping plant.
The disposable knives of disc chippers are typically replaced either by detaching the knife directly from its holding elements in the chipper disc, or, in addition to loosening the holding element, by releasing some other part as well, usually the knife holder. The old knife is extracted from its seat between the tool carrier and the tool holder and replaced with a new one, whereupon the loosened holding elements are tightened again.
Publication FI 78412 presents a reversible-bit-type disposable knife. The knife is replaced at the chipper by loosening the screws in the appropriate part of the chipper disc, drawing out the knife longitudinally, inserting a new knife and tightening the screws. U.S. No. Pat. 3,542,302 discloses a traditional regrindable knife. When mounting a reground knife, the correct chipping height is set by means of setting elements.
In addition to the above-mentioned procedures, other techniques for faster replacement of the knives of a disc chipper have been developed. Among these is a hydraulic knife releasing method, presented e.g. in Finnish application no. 903733, which involves the use of a hydraulic thrust device to produce a force for the loosening of the knife holding elements.
Also previously known is the use of the centrifugal force for the holding of the knifes, but the magnitude of the centrifugal force alone is not sufficient because of the lower speed of rotation of the chipper disc. Employing the centrifugal force would require the use of extra large masses or transmission systems increasing the force. Use of the centrifugal force alone for holding the knives is applicable e.g. to the drumlike chipper disc of a high-speed planer, in which the knives are placed on the periphery of the chipper disc and which allows the use of fairly large masses on the periphery of the drum, the effect of the centrifugal force being applied to these masses to hold the knives.